Pat Mirenda has an article that was published in 1993 that describes the impact of pictures and signs on natural speech. You can get the article from the ASHA.org website if you are a certified SLP. Natural speech shows an increase with both forms of AAC. This is an extremely difficult topic and it is so frustrating when a child is not permitted to use AAC to help him make communication easier! Once communication becomes reinforcing speech output becomes motivating! It makes sense to me!